If the air filter is dirty it will reduce the air flow and cause condensation to form on the interior parts of the air conditioner. This water will then drip inside the motorhome. Make certain the foam filter is clean.
If you don't want to wash and re-use the foam filter, Walmart sells replacement filter material. It is usually located near the home window air conditioners and costs 99 cents. One package has enough to make several filters when cut to fit the Dometic roof A/C.
2010 Georgetown 373
2013 Jeep Wrangler